1. Field of the Invention
The present invention relates to non-aqueous inkjet inks containing specific quinacridone pigments and dispersion synergists.
2. Description of the Related Art
Pigment dispersions are made using a dispersant. A dispersant is a substance for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. Dispersants are generally surface-active materials having an anionic, cationic or non-ionic structure. The presence of a dispersant substantially reduces the required dispersing energy. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersing operation, due to mutual attraction forces. The use of dispersants also counteracts this re-agglomeration tendency of the pigment particles.
The dispersant has to meet particularly high requirements when used for inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of the pigment particles through the nozzles of the print head in an inkjet printer, which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided during the standby periods of the printer.
Many polymeric dispersants contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains sticking out whereby pigment particles are made compatible with the dispersion medium, i.e., stabilized.
In aqueous inkjet inks, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium.
The preparation of good thermally stable dispersions with submicron particles is more difficult for non-aqueous inkjet inks, such as solvent based, oil based and radiation curable inkjet inks. The pigments are especially difficult to disperse when they have a non-polar surface.
These problems have led to the design of very specific polymeric dispersants wherein the anchor groups are pigment derivatives. For example, EP 0763378 A (TOYO INK) discloses a pigment composition including a non-aqueous type pigment dispersing agent having a portion which has a high affinity with a pigment and which has at least one type selected from the group consisting of an organic dye, anthraquinone and acridone only at a terminal end or at both terminal ends of at least one polymer selected from a linear urethane polymer and a linear acrylic polymer, and a pigment.
Another approach for dispersing pigments with non-polar surfaces in non-aqueous dispersion media is changing the surface to a more polar surface by addition of compounds known as dispersion synergists. A dispersion synergist is a compound that promotes the adsorption of the polymeric dispersant on the surface of the pigment. It is suggested that the synergist should possess the pigment structure substituted by one or more sulphonic acid groups or salts thereof.
U.S. Pat. No. 4,461,647 (ICI) discloses a dispersion of a pigment in an organic liquid containing a water-insoluble asymmetric disazo compound including a central divalent group free from acidic and other ionic substituents linked through azo groups to two monovalent end groups characterized in that one end group, the first, is free from acidic and other ionic substituents and the other end group, the second, carries a single substituted ammonium-acid salt group.
U.S. Pat. No. 4,057,436 (ICI) discloses pigment dispersions in organic liquids using polymeric or resinous dispersing agents in the presence of a substituted ammonium salt of a colored acid wherein there are between 16 and 60 carbon atoms contained in at least 3 chains attached to the N-atom of the substituted ammonium ion.
U.S. Pat. No. 6,641,655 (AVECIA) discloses the use of a diquaternary ammonium salt of a colored acid as fluidizing agent wherein the diquaternary ammonium cation contains two or more nitrogen atoms.
Although these dispersion synergists work fine for some pigments, many other pigments can not be dispersed to an acceptable quality in a non-aqueous medium. This is the case for quinacridone pigments, for which it is difficult to obtain stable non-aqueous inkjet inks.
Sulfonated quinacridone derivatives have been disclosed, for example, by U.S. Pat. No. 6,152,968 (BASF). However, U.S. Pat. No. 6,152,968 (BASF) is silent on their use in non-aqueous media.
U.S. Pat. No. 6,827,775 (CIBA SPECIALTY CHEMICALS) discloses the use of a pigment and its sulfonation product in a solid solution, water and organic solvents such as xylene. However, U.S. Pat. No. 6,827,775 (CIBA SPECIALTY CHEMICALS) is silent on the stability of non-aqueous dispersions of quinacridone type pigments and their sulfonation products.
U.S. Pat. No. 4,844,742 (CIBA GEIGY) discloses the surface modification of quinacridone pigments, wherein the pigment is treated with a solvent-free, basic, nitrogen-containing copolymer of the polyurethane series and with a dispersion-improving quinacridone derivative resulting in compositions exhibiting enhanced performance in automotive and other finish systems. The derivatives used are phthalimidomethyl or sulfonic acid derivatives of quinacridone. Again no details are given on the stability of non-aqueous dispersions of quinacridone type pigments.
WO 02/064680 (CLARIANT) discloses quinacridone derivatives including sulfonic acid and carboxylic acid groups used as pigment dispersants in aqueous and non-aqueous inkjet inks and in hotmelt inkjet inks. Radiation curable inkjet inks are not disclosed.
DE 1619618 (DU PONT) discloses quinacridone pigments including up to 25 wt % of quinacridone derivatives including carboxylic acid groups. Pigment dispersions using the quinacridone derivatives are not disclosed.
U.S. 2005/0196697 (KONICA MINOLTA) discloses actinic ray curable compositions containing a quinacridone pigment, a dispersant, a cationically polymerizable monomer, a photo acid generator, and a quinacridone derivative. The exemplified quinacridone derivatives include sulfonamido groups.
U.S. Pat. No. 6,451,103 (TOYO INK) discloses self-dispersible aqueous pigment dispersions of quinacridone pigments with water-soluble pigment derivatives including one or more groups having a carboxylic acid or sulfonic acid or salts thereof. Non-aqueous pigment dispersions are not disclosed.
For consistent image quality, inkjet inks require a dispersion stability capable of dealing with high temperatures (above 60° C.) during transport of the ink to a customer, jetting at elevated temperatures and changes in the dispersion medium of the inkjet ink during use, for example, evaporation of solvent and increasing concentrations of humectants, penetrants and other additives.
Dimethyl substituted quinacridone pigments such as C.I. Pigment Red 122 have been a preferred quinacridone pigment for making inkjet ink sets with a high color gamut, however this quinacridone pigment exhibits poor dispersion quality and stability in comparison to, for example, an unsubstituted quinacridone pigments.
Therefore, it is highly desirable to be able to manufacture inkjet inks using a dimethyl substituted quinacridone pigment in a non-aqueous medium exhibiting high dispersion quality and stability.